1. Technical Field of the Invention
The present invention relates to control of the heating capability of a passenger compartment heater, in an automotive air conditioning system, which can preferably be applied to hybrid vehicles.
2. Description of the Related Art
Conventionally, there are descriptions of engine operation controls to secure a required minimum coolant temperature for a heating function in automotive air conditioning systems that are applied to hybrid vehicles in which a vehicle running engine and a vehicle running motor are installed, and Japanese Patent Publication No. 3323097 describes one of the conventional examples.
In this related art, when a target air-outlet temperature TAO, for air blown out into the passenger compartment, is equal to or higher than a predetermined temperature (for example, 30° C.) and the temperature of an engine coolant, which is a heating medium for heating, is equal to or lower than a set coolant temperature (a required minimum coolant temperature) that is determined based on the target air-outlet temperature TAO, an engine operation demand signal is outputted to an engine control unit so as to force the engine into operation, whereby the temperature of the engine coolant is increased to a higher temperature than the set temperature.
Then, when the engine coolant temperature is higher than the set coolant temperature, an engine stop demand signal is outputted to the engine control unit so as to stop the engine. Thus, the engine coolant temperature is maintained at the required minimum coolant temperature to obtain the target air-outlet temperature TAO.
Incidentally, as a result of the specific test and study of the conventional example specifically, it was found that comfortable air conditioning was interrupted for the following reasons. Namely, in the conventional example, the set temperature required to obtain the target air-outlet temperature TAO is uniformly determined based only on the target air-outlet temperature TAO regardless of change in outlet mode. Consequently, with the same target air-outlet temperature TAO, the set coolant temperature remains at the same temperature whether a foot mode or a bilevel (B/L) mode is used.
Here, in contrast with the foot mode in which air is delivered only to the floor and is hence directed toward the feet of occupants, in the bilevel mode, as air is directed toward both the feet and faces of the occupants, it is desired, for the sake of comfort, to have a so-called face-cool, foot-warm outlet temperature distribution by setting the face outlet temperature lower than the foot outlet temperature.
Moreover, as the set coolant temperature is the minimum coolant temperature required to obtain the target air-outlet temperature TAO, the face-cool, foot-warm outlet temperature distribution cannot be established when the bilevel mode is used for the following reason. Namely, when attempting to set the face outlet temperature lower than the foot outlet temperature by a predetermined temperature in the bilevel mode, as the set coolant temperature is the minimum coolant temperature required to obtain the target air-outlet temperature TAO, even in the event that the foot outlet temperature reaches the target air-outlet temperature TAO, the face outlet temperature inevitably becomes lower than the target air-outlet temperature TAO by the predetermined temperature.
According to this fact, even in the event that the face-cool, foot-warm outlet temperature distribution can be established, then average temperature for the foot outlet temperature and the face outlet temperature becomes lower than the target air-outlet temperature TAO, and therefore, the passenger compartment temperature cannot be set to the set temperature. As a result, in reality, during a bilevel mode operation, air delivered from the foot outlet and air delivered from the face outlet are delivered at the same temperature, that is, the target air-outlet temperature TAO, and this lowers the comfort level provided during the bilevel mode operation.
Then, while it is possible to simply increase a target coolant temperature, that is determined based on a target air-outlet temperature TAO, by a predetermined temperature, in the event that this idea is actually put into practice, the engine coolant is heated to a higher temperature than a minimum coolant temperature required to obtain the target air-outlet temperature TAO during a foot mode operation, for example. This leads to a longer operating time of the engine and worsens the fuel economy thereof, and eventually, the advantage inherent in hybrid vehicles is lost.